It is generally known that antenna performance is dependent upon the size, shape and material composition of the constituent antenna elements, as well as the relationship between certain antenna physical parameters (e.g., length for a linear antenna and diameter for a loop antenna) and the wavelength of the signal received or transmitted by the antenna. These relationships determine several antenna operational parameters, including input impedance, gain, directivity, signal polarity and the radiation pattern. Generally for an operable antenna, the minimum physical antenna dimension (or the electrically effective minimum dimension) must be on the order of a quarter wavelength (or a multiple thereof) of the operating frequency, which thereby advantageously limits the energy dissipated in resistive losses and maximizes the energy transmitted. Quarter wavelength and half wavelength antennas are the most commonly used.
The burgeoning growth of wireless communications devices and systems has created a substantial need for physically smaller, less obtrusive, and more efficient antennas that are capable of wide bandwidth or multiple frequency-band operation, and/or operation in multiple modes (i.e., selectable radiation patterns or selectable signal polarizations). Smaller packaging of state-of-the-art communications devices may not provide sufficient space for the conventional quarter and half wavelength antenna elements. Thus physically smaller antennas operating in the frequency bands of interest and providing the other desirable antenna operating properties (input impedance, radiation pattern, signal polarization, etc.) are especially sought after.
As is known to those skilled in the art, there is a direct relationship between physical antenna size and antenna gain, at least with respect to a single-element antenna, according to the relationship: gain=(βR)^2+2βR, where R is the radius of the sphere containing the antenna and β is the propagation factor. Increased gain thus requires a physically larger antenna, while communications device manufacturers and users continue to demand physically smaller antennas. As a further constraint, to simplify the system design and strive for minimum cost, equipment designers and system operators prefer to utilize antennas capable of efficient multi-frequency and/or wide bandwidth operation, allowing the communications device to access various wireless services operating within different frequency bands from a single antenna. Finally, gain is limited by the known relationship between the antenna frequency and the effective antenna length (expressed in fractional wavelengths). That is, the antenna gain is constant for all quarter wavelength antennas of a specific geometry i.e., at that operating frequency where the effective antenna length is a quarter wavelength of the operating frequency.
The known Chu-Harrington relationship relates the size and bandwidth of an antenna. Generally, as the size decreases the antenna bandwidth also decreases. But to the contrary, as the capabilities of handset communications devices expand to provide for higher data rates and the reception of bandwidth intensive information (e.g., streaming video), the antenna bandwidth must be increased.
One basic antenna commonly used in many applications today is the half-wavelength dipole antenna. The radiation pattern is the familiar omnidirectional donut shape with most of the energy radiated uniformly in the azimuth direction and little radiation in the elevation direction. The typical gain is about 2.15 dBi. Frequency bands of interest for certain communications devices are 1710 to 1990 MHz and 2110 to 2200 MHz. A half-wavelength dipole antenna is approximately 3.11 inches long at 1900 MHz, 3.45 inches long at 1710 MHz, and 2.68 inches long at 2200 MHz.
The quarter-wavelength monopole antenna positioned above a ground plane is derived from a half-wavelength dipole. The physical antenna length is a quarter-wavelength, but since the ground plane (ideally an infinite ground plane) produces an image antenna element the performance resembles that of a half-wavelength dipole. Thus the radiation pattern for a monopole antenna above a ground plane is similar to the half-wavelength dipole pattern, with a typical gain of approximately 2 dBi. It is known that for portable wireless radio equipment a monopole antenna mounted perpendicular to a conducting finite ground plane provides an antenna having good radiation characteristics, a driving point impedance that can be matched to the radio circuitry and relatively simple construction. As compared to a common dipole, the monopole is also smaller in size.
However, as mentioned above, reducing antenna size reduces the operational bandwidth due to the functional relationship between input impedance and frequency. The bandwidth reduction is caused by combination of lower radiation resistance due to the smaller antenna size and a larger amount of stored energy, creating a high Q antenna bandwidth and lower radiation bandwidth. One technique for overcoming the bandwidth limitation, especially applicable to a monopole antenna, surrounds the radiating element with a sleeve. The sleeve extends the ground plane, forming a virtual feed point along the radiating element, thereby extending the antenna bandwidth.
The common free space (i.e., not above ground plane) loop antenna (with a diameter of approximately one-third the wavelength) also displays the familiar donut radiation pattern along the radial axis, with a gain of approximately 3.1 dBi. At 1900 MHz, this antenna has a diameter of about 2 inches. The typical loop antenna input impedance is 50 ohms, providing good matching characteristics. However, conventional loop antennas are too large for handset applications and do not provide multi-band operation. As the loop length increases (i.e., approaching one free-space wavelength), the maximum of the field pattern shifts from the plane of the loop to the axis of the loop. Placing the loop antenna above a ground plane generally increases its directivity.
Printed or microstrip antennas are constructed using the principles of printed circuit board techniques, where a top metallization layer overlying a dielectric substrate serves as the radiating element. These antennas are popular because of their low profile, the ease with which they can be fabricated and a relatively low fabrication cost. One such antenna is the patch antenna, comprising in stacked relation, a ground plane, a dielectric substrate, and a radiating element overlying the top substrate surface. The patch antenna provides directional hemispherical coverage with a gain of approximately 3 dBi. Although small compared to a quarter or half wavelength antenna, the patch antenna has relatively poor radiation efficiency, i.e., the resistive return losses are relatively high within its operational bandwidth. Also, disadvantageously, the patch antenna exhibits a relatively narrow bandwidth. Multiple patch antennas can be stacked in parallel planes or spaced-apart in a single plane to synthesize a desired antenna radiation pattern that may not be achievable with a single patch antenna.
Given the advantageous performance of quarter and half wavelength antennas, many wireless devices employ such antennas. Many wireless devices use a monopole antenna, where the antenna length is on the order of a quarter wavelength of the radiating frequency and the antenna is disposed over a ground plane. These dimensions allow the antenna to be easily excited and operated at or near a resonant frequency, while limiting the energy dissipated in resistive losses and maximizing the transmitted energy. But, as the operational frequency increases/decreases, the operational wavelength correspondingly decreases/increases. Since the monopole antenna over a ground plane should ideally present an electrical length that is a quarter wavelength at the operational frequency, when the operational frequency changes the antenna is no longer operating at a resonant condition and antenna performance deteriorates.
As can be inferred from the above discussion of various antenna designs, each exhibits know advantages and disadvantages. The dipole antenna has a reasonably wide bandwidth and a relatively high antenna efficiency (or gain). The major drawback of the dipole, when considered for use in personal wireless communications devices, is its size. At an operational frequency of 900 MHz, the half-wave dipole comprises a linear radiator of about six inches in length. Clearly it is difficult to position such an antenna in the small space envelope associated with today's handheld devices. By comparison, the patch antenna or the loop antenna over a ground plane present a lower profile antenna structure than the dipole, but as discussed above, operate over a narrower bandwidth with a highly directional radiation pattern.
As discussed above, multi-band or wide bandwidth antenna operation is especially desired for use with various personal or handheld communications devices. One approach to producing an antenna having multi-band capability is to design a single structure (such as a loop antenna) and rely upon the higher-order resonant frequencies of the loop structure to obtain a radiation capability in multiple frequency bands.
Another known method for achieving multi-band performance uses two separate spaced-apart antennas with coupled inputs or feeds for signal splitting according to methods well known in the art. Each of the two antennas resonates at a predictable frequency to provide operation in at least two frequency bands. Certain wireless devises thus employ two or more relatively narrowband antennas to cover a frequency range of interest at the expense of requiring additional space within or proximate the wireless device.
In high signal scattering environments in which wireless devices typically operate, such as office buildings and urban environments, signal fading is a common problem. The signal is reflected from the atmosphere and structures along the path from the transmitter to the receiver, creating multiple received signals, each traversing a different path length. Thus at the receiver, the signals are typically not in phase synchronism, and when coherently combined at the antenna, signal cancellation (i.e., destructive interference) causes a signal fading effect. Such signal fading can be overcome by using two or more antennas to achieve spatial antenna diversity. If the antennas are designed for maximum isolation, then the signals received at each antenna can be considered statistically independent and the likelihood of signal fading is reduced. If spatial and frequency diversity are desired, two sets of antennas are required for each frequency band, with one set providing diversity reception in each band. Clearly, such schemes consume an inordinate amount of space. Further, the degree of diversity provided is functionally related to the antenna spacing. Thus greater diversity requires greater spacing between the antennas and a physically larger antenna system.
Broadband monopole antennas are known in the art and generally comprise solids of rotation oriented with the axis of rotation perpendicular to the ground plane. Examples of such monopole antennas include: a discone antenna, a cylinder over a ground plane, a monopole antenna on a large sleeve (as described above), a top-loaded monopole antenna, a non-circular monopole antenna, an ellipsoidal monopole antenna, and a helical antenna over a ground plane. Several such antennas are described in VHF and UHF Antennas, by R. A. Burberry, published by Peregrinus, 1992.
Each of the many antenna configurations discussed above has certain advantageous features, but none offer all the performance requirements desired for handset and other wireless applications, including dual or multi-band operation, high radiation efficiency, high gain, low profile and low fabrication cost. Thus notwithstanding the many known techniques for achieving the desired antenna performance, it remains difficult to realize an efficient antenna or antenna system that satisfies the multi-band/wide bandwidth operational features in a relatively small physical volume.